JPH06130359A - Liquid crystal display element and compensating element - Google Patents

Abstract

(57) [Summary] [Structure] In a liquid crystal display device in which a driving liquid crystal cell 3 is arranged between two polarizing plates 1 and 4, two polarizing plates are provided between the polarizing plate 1 and the driving liquid crystal cell 3. A compensation cell 3 having a chiral nematic liquid crystal sandwiched between the substrates is arranged. At least one substrate surface of the compensating cell has an angle (α 0) formed by the liquid crystal molecules and the substrate surface of 70 ° ≦ α 0 <90.
A liquid crystal display device characterized by being subjected to uniform liquid crystal alignment treatment so as to be at an angle of. As this compensation cell,
A polymer liquid crystal film may be used. [Effect] A high-quality liquid crystal display device having improved viewing angle characteristics and excellent visibility can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a high contrast ratio and controlled viewing angle dependence, and a compensating device used for this device.

[0002]

2. Description of the Related Art In recent years, liquid crystal display elements, which have the great advantages of thinness, light weight and low power consumption, have been actively used as display devices for personal OA equipment such as Japanese word processors and desktop personal computers.
Most of liquid crystal display elements (hereinafter abbreviated as LCD) use twisted nematic liquid crystal, and as a display method,
It can be roughly classified into two systems, a birefringence mode and an optical rotation mode.

An LCD of a birefringence mode display system using a twisted nematic liquid crystal has a molecular arrangement twisted by 90 ° or more (called ST system) and has steep electro-optical characteristics. Therefore, a switching element is provided for each pixel. Even without a (thin film transistor or diode), a large-capacity display can be easily obtained by time-division driving even with a simple matrix electrode structure.

However, in the ST system, since the birefringence effect is utilized, the display colors are so-called yellow mode display of yellow and dark blue, or so-called blue mode display of white and blue, and black-and-white display and color display are not possible. It was possible. As a means for eliminating such coloring of display, it is possible to realize black and white display by disposing a second liquid crystal cell twisted in the opposite direction between the polarizing plate and the liquid crystal cell.
It is reported in Japanese Patent No. 53528.

On the other hand, the LCD in the optical rotation mode has a molecular arrangement twisted by 90 ° (called the TN mode), has a fast response speed (tens of milliseconds) and shows a high contrast ratio and a good gradation display property. By providing a clock, a calculator, and a switching element for each pixel, a liquid crystal display (TFT-LCD or MIM-LCD) having a large display capacity and high contrast and high display performance can be realized.

However, even in the TN system, the display color and the contrast ratio change depending on the viewing angle and the viewing angle, and the display performance of the cathode ray tube CRT cannot be completely exceeded. As a means for compensating for such viewing angle dependency, a method of disposing a second liquid crystal cell (called UST) sandwiching a chiral nematic liquid crystal between a polarizing plate and a liquid crystal cell is applied, physics, letter (Appl. Phys. Letter) Vol. 60-
15, p. 1806, 1992 (Toshiba: Hato et al.). The chiral nematic liquid crystal has an arrangement in which liquid crystal molecules are twisted in a spiral shape, and a vertically aligned liquid crystal has a positive optical anisotropy, whereas a chiral nematic liquid crystal has a twist amount due to a spiral twist arrangement. When it is large (one rotation or more), it exhibits optically negative optical anisotropy. When the twist angle in the chiral nematic liquid crystal cell is very large (about 10 rotations), its optical anisotropy becomes almost symmetrical. Here, by controlling the twist angle of the chiral nematic liquid crystal cell to some extent, the spiral axis can be tilted from the substrate normal direction. By doing so, the optical anisotropy of the chiral nematic liquid crystal cell can be made asymmetric, and the viewing angle characteristics when a voltage is applied to the driving liquid crystal cell having the asymmetric optical anisotropy can be compensated. It is a thing. However, considering this compensating means more deeply, the optical axis is not substantially tilted, so that the viewing angle dependence of the driving liquid crystal cell having a pre-tilt or a homeotropic or homogeneous alignment driving liquid crystal cell with no pre-tilt is completely compensated. I can't do it.

On the other hand, Japanese Patent Publication No. 3-215831 discloses a means of increasing the pretilt angle of the cholesteric liquid crystal to tilt the optical axis when the twist angle is very large. However, as shown in FIG. 9, most of the liquid crystal molecules in the pretilt-controlled liquid crystal layer when no voltage is applied are below the pretilt, and the optical optical axis (helical axis) does not practically tilt.

[0008]

It is generally known that liquid crystal molecules have different refractive indexes in the major axis direction and the minor axis direction of the liquid crystal molecules. When light of a certain polarization state enters the liquid crystal molecule exhibiting such anisotropy of refractive index, the light changes its polarization state depending on the angle of the liquid crystal molecule. Therefore, depending on the angle of the liquid crystal cell, the display pattern may appear to be reversed, the display pattern may not be visible at all, or the display may be colored depending on whether the light enters the liquid crystal cell vertically or obliquely. It appears as a phenomenon and is not preferable for practical use.

The present invention aims to solve the above-mentioned inconvenience.

[0010]

The present invention comprises a driving liquid crystal cell having a liquid crystal layer between two substrates provided with electrodes, and two polarizing plates sandwiching the driving liquid crystal cell. In the liquid crystal display element, there is at least one compensating cell in which a chiral nematic liquid crystal is sandwiched between two substrates between the at least one polarizing plate and the driving liquid crystal cell. At least one substrate surface of the compensation cell has an angle (α0) formed by the liquid crystal molecules and the substrate surface.
The liquid crystal display element is characterized in that uniform liquid crystal alignment treatment is performed so that 70 ° ≦ α 0 <90 °.

Further, the present invention is characterized in that a compensating element, which is a polymer liquid crystal film having a molecular arrangement for obtaining a compensating effect optically equivalent to that of the compensating cell, is used.

[0012]

In general, there are roughly two types of arrangements of polarizing plates used in liquid crystal display elements. Light is not transmitted when a voltage is not applied to the liquid crystal cell, and light is transmitted when a voltage is applied ( There are a normally-closed method and a normally-open method in which light is transmitted when a voltage is not applied to the liquid crystal cell and light is blocked when a voltage is applied (normally open). As an example, FIG. 3 shows the contrast ratio dependence of the conventional TN type normally open and normally closed when the display surface is inclined from 0 ° to 60 ° in the left and right directions from the normal. It is indicated by (10.0) in the case of normally open and (11.0) in the case of normally closed. Comparing these, it can be seen that normally closed has less viewing angle dependence of contrast ratio than normally open. The contrast ratio is a value obtained by dividing the luminance in a state where light is transmitted (bright state) by the luminance in a state where light is blocked (dark state), and the contrast ratio greatly affects the luminance in a dark state. Then, when the viewing angle dependency in the left-right direction of the luminance in the dark state of both the normally open type and the normally closed type is measured, the characteristics shown in FIG. 4 are obtained. The case of normally open is shown in (10.1), and the case of normally closed is shown in (11.1). As is clear from the figure, normally closed has less viewing angle dependence in the dark state than normally open, and as a result, normally closed has better viewing angle characteristics of contrast ratio than normally open.

Considering the difference between the normally open and normally closed dark states, a voltage is applied to the liquid crystal cell in order to cut off light in the case of normally open, and in the case of normally closed. A voltage is applied to the liquid crystal cell to transmit light. FIG. 5 shows the molecular alignment state when a voltage value that gives a dark state is applied to the liquid crystal cell in the normally open state. Here, 7 and 8 in the figure are tilts, respectively.
Angle and twist (twist) angle, and the tilt angle means the tilt angle 7 of the long axis of the liquid crystal molecule (5.1) with respect to the xy plane when the substrate surface of the liquid crystal cell is the xy plane in the coordinate system shown in FIG. , Twist angle 8 means the liquid crystal molecule (5.1) from the z axis to x
The distance between the two substrates is d, which is the angle formed by the axis projected onto the y-plane and the x-axis.

From FIG. 5, when a voltage is applied, the liquid crystal molecules are tilted near 90 ° near the center of the liquid crystal cell (d / 2), but near the upper and lower substrate surfaces (0, d), The liquid crystal molecules do not tilt much under the influence of the alignment control force. The twist angle has an S-shaped distribution.

When the tilt angle of the liquid crystal molecules is constant with respect to the cell thickness direction and the twist angle is linearly twisted, that is, when the normally-closed dark molecular arrangement state is compared, the molecular arrangement state when voltage is applied is compared. Are viewed differently depending on the viewing angle and orientation of the liquid crystal cell, and as a result, the difference in the appearance of the liquid crystal molecule alignment state is reflected in the viewing angle characteristics as the difference in the polarization state propagating in the liquid crystal cell. Therefore, the reason why the normally closed characteristic is better is that the difference in the appearance of the molecular arrangement state in the dark state depending on the viewing direction angle is smaller than that in the normally open state. Therefore, if devised so that the same molecular arrangement state can be seen from any direction by some method, the viewing angle characteristics in the normally open dark state can be improved.

The largest change in the appearance of the liquid crystal molecules depending on the viewing angle in the liquid crystal cell is where the liquid crystal molecules near the center of the liquid crystal cell are greatly inclined. Therefore, the viewing angle characteristics can be improved by reducing the change in the appearance of this portion.

The alignment state of the liquid crystal can be simply shown by a three-dimensional index ellipsoid. FIG. 7 shows a state in which liquid crystal molecules are vertically erected by a refractive index ellipsoid. The birefringence phenomenon is a difference in refractive index in a two-dimensional plane when the refractive index ellipsoid 6 is viewed from a certain direction. Is a phenomenon related to
The refractive index body (6.4) in the two-dimensional plane when viewed from the z direction (that is, when the liquid crystal cell is viewed from the front) becomes a circle, and the refractive index body when viewed from the refractive index difference and the visual axis (6.1). Different from (6.5). In case of normally open (Cross Nicole),
Since the difference in refractive index when viewed from the z direction is 0, a dark state is obtained, but the two-dimensional surface when viewed from the visual axis (6.1) becomes an ellipse, and as a result, a difference in refractive index occurs, so the dark state Don't When the viewing angle (6.3) of the index ellipsoid 6 is increased, the ellipse (6.5) in the two-dimensional plane seen from the visual axis (6.1) becomes n
It becomes larger in the lengthwise direction of 61, and larger transmitted light is observed when viewed from the direction of the visual axis (6.1).

Therefore, in order to optically compensate such a refractive index ellipsoid, the refractive index in the lengthwise direction of n62 increases as the angle (6.3) for viewing the refractive index ellipse increases. If an index ellipsoid as shown in Fig. 8 (that is, an optically anisotropic body with negative optical property) is placed on the visual axis (6.1), the ellipse (6.5) in the two-dimensional plane becomes a sphere and various Even when observed from the direction, the apparent refractive index becomes substantially the same, and the viewing angle characteristic is improved.

In addition to the cholesteric liquid crystal cell, a film (NOC) having a negative optical anisotropy in the thickness direction or a retardation layer formed by displacing the optical axis may be used as a material exhibiting an optically negative optical anisotropy. Film (RF) and the like can be mentioned.

By the way, the above method is effective in a cell having liquid crystal molecules as shown in FIG. 7 at any position of the cell. However, even liquid crystal molecules near the center of the liquid crystal cell when a voltage is applied are not actually vertical. In a TN cell having a molecular alignment state as shown in FIG.
Even if you put etc., it will not become a sphere by the amount of inclination. On the other hand, when a cholesteric liquid crystal cell in which both substrates 10a and 10b shown in FIG. 9 are horizontally aligned is placed, the liquid crystal molecules 11 have a twisted structure due to the chiral function, and thus the film NOC is formed.
Closer to the sphere than to RF. However, the helical axis of the liquid crystal molecules is perpendicular to the substrate, and the compensation medium is incomplete because it is not a compensation medium having an inclination corresponding to the inclination of the liquid crystal molecules of the driving liquid crystal cell.

By the way, when the inventors insert a cholesteric liquid crystal layer having chiral ability into a cell having vertical alignment on one side or both sides, the central part of the liquid crystal layer has a twisted structure and the substrate surface becomes vertical. 10, a planar structure with a spiral or a hybrid structure with a twist is adopted, and at this time, the angle α 0 formed by the liquid crystal molecules on the substrate surface and the substrate surface is not 0 ° or 90 °, but for driving. It has been found that when tilted according to the liquid crystal cell, the spiral axis also tilts accordingly. That is, in FIG. 10, (a) is obtained by vertically aligning both substrates 10a and 10b,
(B) shows the twist of the chiral nematic liquid crystal molecules 11 and the spiral axis 12 when the substrate 10a is vertically aligned and the substrate 10b is horizontally aligned. In the figure, the extension direction of the liquid crystal molecules 11 in contact with at least one of the substrates 10a, that is, the angle α0 formed by the helix axis 12 and the substrate surface is 70.
It is inclined at ° ≦ α 0 <90 °. Accordingly, by controlling the compensating cell of the present invention so as to correspond to the inclination of the spiral axis of the driving liquid crystal cell, the refractive index ellipsoid after compensation becomes a sphere and the viewing angle dependency can be improved.

Needless to say, the above-mentioned cholesteric liquid crystal cell can obtain the same function by using a polymer liquid crystal layer having a twisting property. In this case, for example, at least one of the substrates of the driving liquid crystal cell is used. The liquid crystal display device is obtained by applying such a polymer liquid crystal layer, is ready for production, and is a more desirable liquid crystal display device. In this case, it is possible to use, for example, a high molecular copolymer liquid crystal having a polysiloxane main chain and having biphenylbenzoate and cholesteryl groups in a side chain at an appropriate ratio.

Although the TN liquid crystal cell has been described above as an example, similar effects can be obtained not only in the TN system but also in a vertically aligned birefringent liquid crystal cell (VAN) having a pretilt angle when a voltage is applied.

[0024]

EXAMPLES Examples of the liquid crystal display device of the present invention will be described in detail below.

(Embodiment 1) FIGS. 1 and 2 show the cell structure in this embodiment. The liquid crystal display element has two polarizing plates 1, 4
And a liquid crystal cell for compensation 2 and a liquid crystal cell for driving 3 between them.
It has a configuration sandwiching between and. The polarizing plate 1 is formed by attaching the polarizing film 1b to the inside of the transparent substrate 1a, and the polarizing plate 4 is also formed by attaching the polarizing film 4b to the transparent substrate 4a. The light transmission axes (1.1) and (4.1) of these polarizing plates 1 and 4 are arranged so as to be orthogonal to each other.

The compensating liquid crystal cell 2 comprises these polarizing plates 1, 4
A chiral nematic liquid crystal layer (Merck Japan, S-81, which is disposed between the two transparent substrates 2a and 2b).
1) It has a liquid crystal cell structure with 2c interposed. Further, the substrate surfaces of the transparent substrates 2a and 2b are vertically aligned.

The driving liquid crystal cell 3 is arranged between the compensating liquid crystal cell 2 and the polarizing plate 4. The upper substrate 3a and the lower substrate 3b form a space between the transparent electrodes 3c and 3d, respectively, and are connected to the driving power supply 3f. The twisted nematic liquid crystal layer 3e is introduced between the substrates 3a and 3b at a twist angle of 90 °, and the state changes according to the applied voltage from the driving power source 3f.

The optical axis of the liquid crystal of the driving liquid crystal cell 3 is twisted counterclockwise from the lower substrate 3b to the upper substrate 3a (left twist). (3.1) and (3.2) are rubbing axes of the upper and lower substrates, respectively, which are orthogonal to each other. The thickness of the liquid crystal layer of the driving liquid crystal cell 3 is 6.0 μm.

The compensating liquid crystal cell 2 has a twist angle of 5850.
Liquid crystal cell with a thickness of 6.0 μm and shown in (2.1) and (2.2)
Are rubbing axes of the upper and lower substrates 2a and 2b, respectively, which are orthogonal to each other. The substrates 2a and 2b are obtained by rubbing a vertical alignment layer as an alignment film, and the angle α0 formed by the substrate surface and the liquid crystal molecules is 85 ° on the vertical side.
The horizontal side is 5 °.

Refractive index n in the thickness direction of the compensating liquid crystal cell
2c is 1.489, and the refractive index n1c in the in-cell direction is 1.
524 (retardation value = −0.228 μm),
The refractive index n2 of the driving liquid crystal cell in the dark state (when 5 V is applied between the liquid crystal cell electrodes from the driving power supply 3f) is the same as the refractive index n2 in the in-cell direction and the refractive index n1 in the thickness direction (n1.
= N1c, n2 = n2c).

The transmission axis (1.1) of the polarizing plate 1 is parallel to the rubbing axes (2.1) and (3.1) of the upper substrate, and the transmission axis (4.1) of the polarizing plate 4 is used.
And the rubbing axes (2.2) and (3.2) of the lower substrate are parallel.

FIG. 12 shows isocontrast curves of the liquid crystal display element of this embodiment. The horizontal axis represents the inclination of the observation point with respect to the vertical direction of the substrate 10 as the viewing angle θ as shown in FIG. 11, and the horizontal direction of the substrate. It is the iso-contrast characteristic when observing points are placed at various positions when the azimuth angle from the direction is φ. Each curve has a contrast ratio CR of 5: 1, 10: 1,
In the cases of 30: 1 and 50: 1, the viewing angle dependence of the contrast ratio is obviously improved as compared with the isocontrast characteristic of the conventional element without the optical anisotropic element shown in FIG.

(Embodiment 2) In Embodiment 1, the substrate surfaces of both transparent substrates 2a and 2b are vertically aligned, and Embodiment 1
Arranged in the same way. The angle α 0 formed by the planes of the substrates 2a and 2b and the liquid crystal molecules at that position is 85 °. When the electro-optical characteristics were measured, the same characteristics as in Example 1 were obtained, and 1
When a 0-inch TFT-LCD was made with this configuration and displayed in 16 gradations, a high contrast LCD capable of distinguishing between 16 gradations even when the viewpoint was changed was realized. When the viewing angle characteristics were measured, the contrast ratio was 4 at 30 ° cone.
A value of 0: 1 or more was obtained, and a good display without inversion of the display surface or change in display color was obtained even when the incident angle was 60 ° or more.

(Comparative Example 1) In Example 1, the viewing angle characteristics of the liquid crystal display element in the case where the compensating liquid crystal cell 2 was not arranged between the driving liquid crystal cell 3 and the upper polarizing plate 1 were measured. The isocontrast curve of the measurement result is shown in FIG. From the figure, it can be seen that the contrast ratio depends on the viewing angle depending on the viewing angle.

(Comparative Example 2) In Example 1, the transparent substrates 2a and 2b were not subjected to the vertical alignment treatment, and were arranged in the same manner as in Example 1 to measure the viewing angle characteristics of the liquid crystal display element. The isocontrast curve of the measurement result is shown in FIG. Although improved from FIG. 13, compared with FIG. 12, 0 °, 90 °, 1
The viewing angle characteristic of the contrast ratio in the 80 ° azimuth is inferior.

(Example 3) In Example 1, the substrate 2
The alignment treatment between a and 2b was vertical alignment with tilt, n-type liquid crystal was introduced in the same manner as in Example 1, and the same arrangement as in Example 1 was performed. A birefringence control type liquid crystal element (ECB) of 640 × 480 dots was created with this configuration, and when simple multiplex driving was performed at 1/240 duty, a contrast ratio of 23: 1 or more was obtained with a 40 ° cone and the incident angle was Even at 60 ° or more, good display without reversing the display surface or changing the display color was obtained.

Example 4 In Example 2, the substrate 2
The alignment treatment between a and 2b was vertical alignment with tilt, and an n-type liquid crystal was introduced in the same manner as in Example 1 to form a compensation cell, which was arranged in the same manner as in Example 2. An ECB type liquid crystal device with 640 x 480 dots was created with this configuration, and simple multiplex drive at 1/240 duty yielded a contrast ratio of 21: 1 or higher at a 40 ° cone and displayed even at an incident angle of 60 ° or higher. Good display was obtained without surface reversal and display color change.

(Embodiment 5) Instead of the compensating cell 2 of Embodiment 1, a polymer material such as polyethylene is used, and the molecules on both sides have an inclination of the helix axis with α 0 of 85 °. A polymer liquid crystal film having the same characteristics as the compensating element 2 was arranged between the driving liquid crystal element 3 and the optical rotation plate 1. When the isocontrast ratio-azimuth characteristic was examined in the obtained configuration,
The visibility was improved as in Example 1.

[0039]

According to the present invention, it is possible to provide a liquid crystal display device of high quality display in which the viewing angle characteristics of the liquid crystal display device are improved and the visibility is excellent. Needless to say, even if the present invention is applied to an active matrix liquid crystal display element using a 3-terminal or 2-terminal element such as TFT or MIM, excellent effects can be obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a liquid crystal display element of Example 1 of the present invention.

FIG. 2 is an exploded perspective view showing the configuration of the liquid crystal display element according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a viewing angle characteristic of a contrast ratio between a normally open and a normally closed type in a left-right direction of a conventional TN type liquid crystal display device.

FIG. 4 is a diagram illustrating a viewing angle characteristic of brightness in a normally open and normally closed dark state of a conventional TN type liquid crystal display device.

FIG. 5 is a diagram showing a molecular arrangement in a thickness direction of a liquid crystal cell when a voltage is applied to the liquid crystal cell.

FIG. 6 is a diagram showing a coordinate system of tilt angles and twist angles of the liquid crystal molecules of FIG.

FIG. 7 is a diagram showing a three-dimensional refractive index ellipsoid with liquid crystal molecules standing.

FIG. 8 is a diagram illustrating a refractive index ellipsoid for optically compensating the refractive index ellipsoid of FIG.

FIG. 9 is a schematic diagram illustrating a pretilt-controlled liquid crystal layer.

FIG. 10A is a schematic view for explaining a spiral plane of a substrate surface and liquid crystal molecules, and FIG. 10B is a schematic view for explaining a twisted hybrid structure of the substrate surface and liquid crystal molecules.

FIG. 11 is a diagram for explaining a measurement coordinate system of an equal contrast ratio-azimuth characteristic.

FIG. 12 is a diagram illustrating an equal contrast ratio-azimuth angle characteristic according to the first embodiment of the present invention.

FIG. 13 is a diagram for explaining the isocontrast ratio-azimuth characteristics of a conventional element.

FIG. 14 is a diagram illustrating UST-type equal contrast ratio-azimuth angle characteristics.

[Explanation of symbols]

 1, 4 ... Polarizing plate 2 ... Compensating liquid crystal cell 3 ... Driving liquid crystal cell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hitoshi Hato 8 Shinsita-cho, Isogo-ku, Yokohama-shi, Kanagawa Incorporated company Toshiba Yokohama Office

Claims (3)

[Claims] 1. A liquid crystal display device comprising: a driving liquid crystal cell having a liquid crystal layer between two substrates provided with electrodes; and two polarizing plates sandwiching the driving liquid crystal cell. At least one compensating cell having a chiral nematic liquid crystal sandwiched between two substrates is provided between one polarizing plate and the driving liquid crystal cell, and at least one substrate of the compensating cell. The surface has an angle (α 0) formed by the liquid crystal molecules and the substrate surface of 70 ° ≦ α 0 <90.
A liquid crystal display device characterized by being subjected to uniform liquid crystal alignment treatment so as to be at an angle of. 2. The liquid crystal display device according to claim 1, wherein the molecular arrangement on one surface side of the compensating cell is a vertical alignment having a tilt. 3. A polymer liquid crystal film, wherein the molecular arrangement on at least one surface side forms an angle (α
0) is a vertical alignment element having a tilt of 70 ° ≦ α 0 <90 °